This invention relates to a local access network intended for mobile units.
Some mobile unit local access networks already exist, and it may be useful to refer to patent document FR-A-2 714 559, which outlines a similar network consisting of a double bus interconnecting, for example through concentration nodes, several radio stations to which mobile stations, also called mobile units or mobiles can be connected.
In time, communications between mobile units or between a mobile and another party will not be voice only, but may require carrying representational data such as still or animated images, which will justify the use of multiflow or even wideband networks. By design, ATM (Asynchronous Transfer Mode) technology makes it possible to transfer any type of information at variable rates of transmission.
A main purpose of this invention is to offer a mobile unit local access network which supports this ATM technology.
Some mobile unit local access networks consist of a distribution network, itself comprised of several nodes connected to a concentrator, to which is connected at least one radio station to communicate with mobiles associated with this base station (RS). At least one of these nodes of the distribution network is connected, through a switching device, to a virtual channel selector.
The growing demand for communication services between mobiles has resulted in increasing traffic load, which each radio station in the local access network must be able to handle. To ensure better radio coverage of urban areas while taking into account the necessarily limited performance of these radio stations, one has to reduce the coverage of each-radio station and to offer networks based on macrocellular architectures. Nevertheless, this creates the problem of a very significant increase in signaling procedures, especially when mobiles leave the radio coverage of one station and come under the coverage of another station, in what are technically referred to as xe2x80x9chand-over phases.xe2x80x9d The procedures to manage the hand-over phases require some handling processes which might quickly saturate ground-based control units.
French Patent 2,714,559 describes a management procedure for quick hand-over phases in which ATM support-links are established and modified in real time. Based on this procedure, a mobile already linked to an initial station is taken over by a second station it has just reached. At that moment, the new station commands the ATM access network to establish the necessary connection(s) so that the mobile can, both transmit its user traffic to the other parity and, on the other hand, receive the incoming traffic from that same party. All processing required for this operation is done by the network""s peripheral concentrators connected to the first and second stations.
This eliminates the risk of saturation of the hand-over phase processing units and ensures the capacity to manage a very large number of mobiles. Another advantage stems from the fact that virtual circuit switching in ATM executes low-level operations whose processing speeds-ensure extremely fast connections.
The procedure described in French Patent 2,714,559 can, however, show some limitations in certain situations of call traffic overload at one concentrator and/or in cases of extreme variations in radio propagation conditions. Network reaction time may then prove too long for the expected performance and, quality.
The purpose of this invention is to solve this problem and to offer a mobile unit access network of a type comprising multiple radio stations linked, by connectors to a distribution network, with each radio station covering a radio cell within which it can communicate with a mobile unit and each concentrator designed to ensure the virtual connection of at least one station with said distribution network.
For a call from a mobile, an essential feature of this invention provides for the establishment of virtual connections from the network to the stations covering a macrocell consisting of a pre-determined group of radio cells which cover the area in which said mobile is or is likely to move.
This feature makes it possible to anticipate the movements of a mobile due to the fact that the virtual connections required for a call are already established when the mobile comes under a new station, called the change-over station. So, as it moves, the mobile receives its user information without delay.
Consistent with another feature of this invention, each station within the network has an associated, predetermined macrocell as the station becomes the core station of this macrocell, and when a mobile unit leaves the coverage of one stationxe2x80x94the fading stationxe2x80x94and enters the coverage of anotherxe2x80x94the change-over stationxe2x80x94it is designed to establish the virtual connections to the stations covering the macrocell associated with the change-over station and to release the virtual connections belonging to the stations in the macrocell associated with the previous station but not belonging to the macrocell associated with the change-over station.
Thus, a macrocell shifts simultaneously with the mobile.
Appropriately, the release of the virtual connections to the stations in the macrocell associated with the previous station but not belonging to the macrocell associated with the change-over station is delayed by a preset change-over time.
When radio cells covered respectively by stations within said network are spread over a dense geographic area, each macrocell is appropriately covered by a group consisting of the core station of said macrocell and of the stations adjoining to the core station or directly contiguous to the core station. This configuration of each macrocell is designed to accommodate, for example, dense geographic areas such as cities, etc.
When radio cells covered respectively by stations within said network are spread over a linear geographic area, which is, for example, the case with radio cells covering a road or highway, each macrocell is covered by a linear group consisting of the core station of the macrocell and of the stations adjoining to the core station. For example, each macrocell is covered by a group of three adjoining, aligned stations and said core station is the central station. Coverage may also be achieved by a group of two adjoining stations, with said core station being the station located behind the mobile in question, relative to its direction of movement.
Consistent with another feature of the invention, its network includes a table in which are described and recorded the characteristics of each macrocell of said network, inter alia the core station of said macrocell and the adjoining stations forming the macrocell.
Consistent with another feature of the invention, the network includes at least one processor which, upon receiving a request from a mobile just entering the coverage of a new station, called a change-over station, transmits to the appropriate concentrator(s) a message to create and release the virtual connections required for the creation and fading of the concerned macrocells. Such a request includes, inter alia, the identifiers of the virtual channels and paths assigned to the call in question, as well as an image of the previous macrocell.
According to a first process embodiment of the invention, the network includes a processor which is centralized. The request is then transmitted to the concentrator to which the change-over station is linked through a channel permanently established and identified, at the level of said change-over station, by a unique pair of identifiers of virtual paths and channels, the request being transmitted from the concentrator to said central processor over a channel identified both by a virtual channel identifier reflecting the address of said concentrator within the network and by a virtual channel identifier reflecting the address of said station in the concentrator.
According to a first process variant of this embodiment, the message to create and release the virtual connections required for the creation and fading of the concerned macrocells, transmitted by the central processor, is broadcast to all the concentrators in the network, which perform the screening.
According to a second process variant of this embodiment, the message to create and release the virtual connections required for the creation and fading of the concerned macrocells is transmitted point-to-point by said central processor to the concerned concentrator(s).
According to a second process embodiment of the invention, the network includes multiple processors intended, for example in, the concentrators, to create and release the virtual connections required for the creation and fading of the concerned macrocells. The processors may be shared.
According to a first process variant of this embodiment, the network is designed so that the message to create and release the virtual connections required for the creation and fading of the concerned macrocells, from a processor to the concerned concentrator(s), is transmitted over a permanent broadcast channel.
According to a second process variant of this embodiment, the network is designed so that the message to create and release the virtual connections required for the creation and fading of the concerned macrocells, from the processor of each concentrator to another concentrator, is transmitted over a virtual path permanently established between said two concentrators.
According to a third process derivative, it is designed so that the message to create and release the virtual connections required for the creation and fading of the concerned macrocells, from the processor of each concentrator to another concentrator, is transmitted over a virtual path permanently established between all concentrators. The virtual path is, appropriately, a virtual ring in which said message travels from one concentrator to another.
According to another process variant of this embodiment, the message is transmitted as many times as there are concentrators concerned by the creation and fading of the macrocells, each concentrator considering the message only if it must perform a virtual connection marking operation for the creation of the new macrocell or the fading of the previous macrocell.